1. Field of the Invention
The present invention relates to an image-sensing apparatus, and particularly to an image-sensing apparatus incorporating a solid-state image-sensing device having a plurality of pixels arranged therein.
2. Description of the Prior Art
Solid-state image-sensing devices designed for use in image-sensing apparatuses are not only small-sized, light-weight, and power-saving, but also free from image distortion, free from image burn-in, and resistant to unfavorable environmental conditions such as vibration and electric fields. Moreover, such image-sensing devices can be manufactured by a process common or analogous to one used to manufacture LSIs (large-scale integrated circuits), and therefore they are reliable and fit for mass production. For these reasons, solid-state image-sensing devices having pixels arranged in a line are widely used in facsimile machines, flat-bed scanners, and the like, and solid-state image-sensing devices having pixels arranged in a matrix are widely used in video cameras, digital cameras, and the like. Solid-state image-sensing devices are roughly grouped into two types according to the means they use to read out (extract) the photoelectric charges generated in their photoelectric conversion devices, namely a CCD type and a MOS type. A CCD-type image-sensing device accumulates photoelectric charges in potential wells while transferring them, and has the disadvantage of a comparatively narrow dynamic range. On the other hand, a MOS-type image-sensing device reads out electric charges accumulated in the pn-junction capacitances of photodiodes through MOS transistors.
Now, how each pixel is configured in a conventional MOS-type solid-state image-sensing device will be described with reference to FIG. 37. As shown in this figure, a photodiode PD has its cathode connected to the gate of a MOS transistor T101 and to the source of a MOS transistor T102. The MOS transistor T101 has its source connected to the drain of a MOS transistor T103, and this MOS transistor T103 has its source connected to an output signal line VOUT. A direct-current voltage VPD is applied to the drain of the MOS transistor T101 and to the drain of the MOS transistor T102, and a direct-current voltage VPS is applied to the anode of the photodiode.
When light enters the photodiode PD, a photoelectric charge is generated therein, and this electric charge is accumulated at the gate of the MOS transistor T101. Here, when a pulse signal φV is fed to the gate of the MOS transistor T103 to turn this MOS transistor T103 on, a current proportional to the electric charge accumulated at the gate of the MOS transistor T101 flows through the MOS transistors T101 and T103 to the output signal line. In this way, it is possible to read an output current that is proportional to the amount of incident light. After this signal has been read, the MOS transistor T103 is turned off and thereby the MOS transistor T102 is turned on so that the gate voltage of the MOS transistor T101 will be initialized.
As described above, in a conventional MOS-type solid-state image-sensing device, at each pixel, the photoelectric charge generated in the photodiode PD and then accumulated at the gate of the MOS transistor T101 is directly read out. This, however, leads to a narrow dynamic range and thus demands accurate control of the amount of exposure. Moreover, even if the amount of exposure is controlled accurately, the obtained image tends to suffer from flat blackness in dim portions thereof and saturation in bright portions thereof.
On the other hand, the assignee of the present invention has once proposed a solid-state image-sensing device including a light-sensing means that generates a photoelectric current in accordance with the amount of incident light, a MOS transistor to which the generated photoelectric current is fed, and a bias-supplying means that supplies a bias to the MOS transistor to bring it into a state in which a subthreshold current flows therethrough so that the photoelectric current is subjected to logarithmic conversion (refer to U.S. Pat. No. 5,241,575).
In this solid-state image-sensing device, when the individual pixels are reset to their original state after image sensing, except in a low-brightness region, a current (called a reset current) having the opposite polarity to the photoelectric current readily flows into the MOS transistor. Therefore, it is easy for the photoelectric charge accumulated in the MOS transistor to recombine with the reset current, and thus resetting is achieved quickly. However, in a low-brightness region, under the influence of the threshold voltage of the MOS transistor, the reset current has difficulty in flowing into the MOS transistor. Therefore, it is difficult for the photoelectric charge accumulated in the MOS transistor to recombine with the reset current, and thus resetting takes an unduly long time. In this way, in a low-brightness region, the individual pixels exhibit poor response, and this causes an after-image to appear in immediately subsequent image sensing.